Mixed Strand Spaces

Strand space analysis [13, 12] is a method for stating and proving correctness properties for cryptographic protocols. In this paper we apply the same method to the related problem of mixed protocols, and show that a protocol can remain correct even when used in combination with a range of other protocols. We illustrate the method with the familiar OtwayRees [10, 1] protocol. We identify a simple and easily verified characteristic of protocols, and show that the OtwayRees protocol remains correct even when used in combination with other protocols that have this characteristic. We also illustrate this method on the NeumanStubblebine protocol [9]. This protocol has two parts, an authentication protocol (I) in which a key distribution center creates and distributes a Kerberos-like key, and a reauthentication protocol (II) in which a client resubmits a ticket containing that key. The re-authentication protocol II is known to be flawed [2]. We show that in the presence of protocol II, there are also attacks against protocol I. We then define a variant of protocol II, and prove an authentication property of I that holds even in combination with the modified II.